Magnetorheological (MR) fluids are disclosed. The present invention uses the initial steps of a sol-gel process to assist in preparation of magnetorheological fluids that have significantly enhanced stability and redispersibility
Most magnetic metal and ceramic powders used in preparation of magnetorheological (MR) fluids and other applications actually have a surface that is not pure metallic or ceramic. The presence of oxygen and hydrogen on the surface of metal or ceramic powders is not unexpected. Many otherwise pure or nearly pure metals are expected, based on their thermodynamic tendency and kinetics of chemical reactions, to undergo chemical corrosion when they are exposed to atmospheres that contain oxygen gas (e.g. air). Similarly, the surface of ceramic particles can be hydroxylated. This observation has been documented in the prior art.
Iron powders have been used for formulating many MR fluid compositions that have practical applications in devices such as dampers, clutches, and brakes. The surface of most iron powders comprises metallic iron and thin layers of an oxide like material. The x-ray photoelectron spectra for various types of iron powders show that, in addition to metallic iron, the surface (probably a few nanometers of the surface region) also consists of oxygen and hydrogen (probably in form of oxygen ions and hydroxyl ions). The exact form of chemical species and their concentration is very difficult to measure. Many MR fluids have been developed which take advantage of the hydrogen-bonding ability of the surface species.
U.S. Pat. No. 5,645,752 (Weiss, et al.) discloses the use of ultrafine oxide materials such as silica whose surface has been modified with a polymer, such as siloxanes. The surface modified oxide particles are added to the magnetic particles and form, through hydrogen bonding, a thixotropic network that helps minimize settling.
U.S. Pat. No. 5,578,238 (Weiss, et al.) discloses MR materials utilizing surface modified particles; surface contaminants are removed so as to improve the magnetic performance of the MR fluids. This is done by an abrader or chemical treatment.
U.S. Pat. No. 5,667,715 (Foister) and U.S. Pat. No. 6,149,832 (Foister) disclose MR fluids comprising solid magnetic particles in which a portion of the volume fraction of the particles is comprised of relatively large particles and a second portion of the volume fraction is comprised of relatively small particles. The carrier fluids disclosed are polyalphaolefins and glycol esters, highly non-polar fluids which ensure that the carbonyl iron particles form hydrogen bonds with other particles and not with any hydrogen in the fluid.
See also the inventor""s prior patent, U.S. Pat. No. 5,985,168, expressly incorporated herein by reference.
There is a continued need for the development of MR fluids having improved dispersability.
The present invention solves the above need by making use of the surface species found on the magnetic particles, in contrast to prior art methods in which these are viewed as contaminants. The invention provides an magnetorheological (MR) fluid comprising soft magnetic particles having a surface that is or can be hydroxylated, a sol-gel precursor that is capable of inducing sol-gel and cross-condensation reactions with the surface of the magnetic particles, and a carrier liquid.
The MR fluids of the present invention have better redispersibility, through the use of the sol-gel process, to produce stable magnetorheological (MR) fluids. This is accomplished through the use of precursors to chemically alter the naturally occurring surface of the magnetic material. No cleaning of particle surfaces (as recommended in the Weiss Patent (U.S. Pat. No. 5,578,238)) is necessary. The interaction involves formation of a covalent bond between the precursor and magnetic particle. Once such a reaction occurs, the precursor is essentially incapable of forming additional hydrogen bonds, in contrast to prior art methods that disclose hydrogen bond formation between the chemicals or particles added. The sol-gel precursors used in the fluids of the present invention do not form a thixotropic network.
It is an object of the present invention, therefore, to provide a stable magnetorheological fluid using the sol-gel process.
It is a further object of the present invention to provide a stable magnetoreheological fluid that does not use hydrogen bonding to maintain dispersibility of the metal particles.